1. Field of the Invention
The present invention relates to a method of fabricating TFTs.
2. Description of Related Art
Thin-film transistors (TFTs) using an amorphous silicon film as an active layer have been well known. TFTs using this amorphous silicon film as an active layer are employed in active matrix liquid crystal displays (AMLCDs).
However, TFTs using an amorphous silicon film have low characteristics and so the P-channel type cannot be put into practical use. Therefore, the actual situation is that the use of the TFTs using an amorphous silicon film is limited to active matrix circuits.
An active matrix liquid crystal display with which peripheral driver circuits are integrated to reduce the fabrication cost and to miniaturize the liquid crystal panel is also known. In this configuration, even the peripheral driver circuits are composed of TFTs. Therefore, this kind of display must meet some requirements, i.e., high-speed operation and realization of P-channel TFTs.
TFTs using a crystalline silicon film are known as a configuration satisfying these requirements. One main method of obtaining a crystalline silicon film is heat treatment for an amorphous silicon film. Another main method is to irradiate the amorphous silicon film with laser light.
The former method results in a clear polycrystalline state. However, the electrical characteristics tend to be nonuniform because of clear crystal grain boundaries. This problem is deeply concerned with the fact that the positions and the state of the crystal grain boundaries cannot be controlled.
The latter method can produce a crystalline silicon film of uniform quality. However, the production yield is low. In addition, the crystallinity is not sufficiently high.
In an attempt to solve these problems, we have proposed a technique for obtaining a crystalline silicon film having desired film quality and electrical characteristics. In particular, nickel is introduced into an amorphous silicon film, and then a heat treatment is performed.
However, doping is utilized in forming source and drain regions. The resulting damage must be healed. Nickel element remaining in the active layer adversely affects the characteristics of the completed device. These problems must be alleviated.